The present invention relates generally to wireless communication systems and, in particular, to methods used in wireless communication systems to locate a mobile terminal.
Estimating the location of a mobile terminal is becoming more and more important in wireless communication systems. For example, the location of a mobile terminal is used in applications such as emergency 911 and location-based billing. A number of geo-location techniques are known for estimating the location of a mobile terminal. A typical one of these geo-location techniques is to estimate and use a round-trip delayxe2x80x94the time it takes a signal to travel from a base station to the mobile terminal and backxe2x80x94to determine the distance of the mobile terminal from the base station. This distance and knowledge of which so-called sector of the cell of the wireless communication system the mobile terminal is located in are then used to determine the location of the mobile terminal. This typically allows the estimation of the location of the mobile terminal with an accuracy of between 200 meters (m) and 1500 m, depending on the size of the cell.
Other geo-location techniques can be used instead of, or in addition to, round trip delay to estimate the location of the mobile terminal. For example, U.S. Pat. No. 6,061,337, entitled xe2x80x9cSystem and Method for CDMA Handoff Using Telemetry to Determine the Need for Handoff and to Select the Destination Cell Sitexe2x80x9d, and U.S. Pat. No. 5,969,679, entitled xe2x80x9cMethod and Apparatus for Determining Whether a Wireless Station is Operating Within a Prescribed Geographic Regionxe2x80x9d, both incorporated herein by this reference, describe geo-location techniques that can be used to estimate the location of the mobile terminal. The location of the mobile terminal can also be estimated using the well-known techniques of time-difference-of-arrival, angle-difference-of-arrival, and/or the angle-of-arrival. Some of these geo-location techniques can be used regardless of the number of base stations visible to the mobile terminal while others cannot be used unless a certain number of base stations are xe2x80x9cvisiblexe2x80x9d to the mobile terminal. A base station is said to be visible to the mobile terminal when its so-called pilot signal is adequately detectable by the mobile terminal. For example, time-difference-of-arrival and round trip delay can be used in combination to determine the location of the mobile terminal when at least two base stations are visible to the mobile terminal. However, to use time-difference-of-arrival alone to determine the location of the mobile terminal at least three base stations should be visible to the mobile terminal.
In some applications the degree of accuracy of the location of the mobile terminal estimated with the various geo-location techniques and/or combinations of same, as just described, is acceptable. In other applications, however, a still higher degree of accuracy is required. In such cases, the estimated location can be used as a parameter in other techniques, for example, wireless assisted GPS (AGPS), to estimate the location of the mobile terminal more precisely, i.e. to refine the estimated location of the mobile terminal. A more accurate estimation of the location of the mobile terminal provided by the geo-location techniques allows for a quicker estimation of the location of the mobile terminal by such techniques as AGPS.
Due to both ever-more-stringent government requirements and to the increased profitability of system operation resulting from improvement in the speed and accuracy of estimating the location of mobile terminals, such improvement is a continuing goal in the wireless communication industry. Thus, while the known geo-location techniques for estimating the location of the mobile terminal are good enough to fill most present requirements, they may not fill proposed government requirements and/or industry desires going forward. Therefore, there is still a need to estimate the location of the mobile terminal more accurately.
The present inventors have realized that further improvement in estimating the location of a mobile terminal is possible. Some geo-location techniques, referred to herein as forward-link geo-location techniques, use only information obtained from one or more forward-link signalsxe2x80x94the signals that are transmitted from the base station to the mobile terminal. Other geo-location techniques, referred to herein as reverse-link geo-location techniques, use information obtained from reverse-link signalsxe2x80x94the signals that are transmitted from the mobile terminal to the base stationxe2x80x94either alone or in combination with information obtained from the forward-link signals. The present invention takes advantage of the present inventors"" recognition that when there are enough base stations visible to the mobile terminal to be able to use the forward-link geo-location techniques, then the reverse-link signals are typically weak. As a result, when there are enough base stations visible to the mobile terminal to be able to use the forward-link geo-location techniques, the reverse-link geo-location techniques may produce results that are less accurate than the results produced by the forward-link geo-location techniques. Thus, even a technique that combines the results of the forward-link and reverse-link geo-location techniques may produce results that are less accurate then the results produced by the forward-link geo-location techniques alone.
In accordance with the present invention, then, the estimation of the location of a mobile terminal is improved by: a) using at least one forward-link geo-location technique without using any reverse-link geo-location techniques, this being performed when there are enough base stations visible to the mobile terminal to be able to use the forward-link geo-location technique without using any reverse-link geo-location techniques to estimate the location, and b) using at least one reverse-link geo-location technique when there are not enough base stations visible to the mobile terminal to be able to use the forward-link geo-location technique without also concurrently using any reverse-link geo-location techniques to estimate the location.
The forward-link geo-location techniques can be used either alone or in combination with each other, and include such known geo-location techniques as a time-difference-of-arrival technique and angle-difference-of-arrival technique.
In an illustrative embodiment of the invention, when the predetermined number, for example three, base stations are visible to the mobile terminal, the location of the mobile terminal is estimated using time-difference-of-arrival. When fewer than three base stations are visible to the mobile terminal, the location of the mobile terminal is estimated using round trip delay.
Estimating the location of a mobile terminal by using only forward-link geo-location techniques when there are enough base stations visible to the mobile terminals to be able to use forward-link geo-location techniques, produces a more accurate estimation of the mobile terminal""s location. It also allows wireless assisted GPS (AGPS) systems to more quickly estimate a more precise location of the mobile terminal.